topping is not really needed in the sea of green method, or if you flowere before 12 inches. Topping is best used when you plan to grow large plants in 5 gal pails or bigger. the effect is to produce 2 or more cola buds instead of one, and also to keep the plant shorter.
Be advised. It has been my expierence that a single large cola will produce more than two smaller ones. Also the bushier the plant gets, the more light you need to finish it.